Fabrics used for manufacturing air bags are required to have, in particular, a low air permeability. Previously, a low air permeability has been mainly achieved by coating the air bag fabric. However, coated fabrics, besides being more expensive to produce, also have appreciable disadvantages in use. In particular, one disadvantage is the increased space required, as compared with uncoated fabrics, for accommodation in the steering wheel.
For this reason, processes for manufacturing uncoated air bag fabrics have been developed. Here, the required low air permeability is achieved through a very tight fabric construction and through special measures in finishing such as calendering (e.g. EP-A-314,867) or heat setting (e.g. CA-A-974,745).
It is true that low air permeability is the most important requirement of an air bag fabric, but a serviceable fabric of this kind must additionally meet a number of other important requirements, of which high strength and good foldability are particularly important. The latter requirement is crucial if the air bag is to be accommodated in the steering wheel of motor vehicles in the least amount of space. However, good foldability also makes possible trouble-free inflation of the air bag for protecting the vehicle occupant in the event of an accident.
To be able to obtain high strengths, air bags have previously been manufactured using, in particular, yarns having a high filament linear density. For instance, U.S. Pat. No. 3,842,583 proposes for this purpose yarns having a filament linear density of 6.0-6.2 den (6.6-6.8 dtex).
CA-A-974,745 proposes a yarn having a filament linear density of 6 den (6.7 dtex).
Although U.S. Pat. No. 4,977,016 specifies a yarn linear density range of 400-600 den (440-660 dtex) for 100-300 individual filaments, the invention is described in the embodiment examples only in terms of a yarn having a linear density of 440 den and 100 individual filaments, which corresponds to a filament linear density of 4.4 den (4.8 dtex).
It is true that these linear densities give the required strength, but they have the appreciable disadvantage of high stiffness, which has a very adverse effect on foldability. The least-space requirement for accommodating the air bag, for example in the steering wheel of the vehicle, and trouble-free inflation cannot be adequately achieved with linear densities of this order of magnitude.
Similarly, very low filament linear densities have been proposed for manufacturing air bag fabrics. For instance, JP-A-64-041,438 proposes to manufacture coated air bag fabric yarns with filament linear densities below 3 den (3.3 dtex). These low linear densities do not cause problems when used for coated fabrics because a tight weaving is not necessary in this case. Normally manufacturing a yarn with a low filament linear density in the man-made fiber industry is more difficult than manufacturing a yarn with a higher filament linear density. As a consequence of more breakages in the spinning and drawing stages, a yarn with a lower filament linear density results in a more fluffy material than a yarn with a higher filament linear density. Especially in view of the tight weaving needed to manufacture uncoated air bag fabrics, a fluffy material results in difficulties and therefore in a lower weaving efficiency.